5 Answers
5

You probably don't want to do it at all. Noninteger numbers are best written in their decimal form (1.5, 5.0). If they can be expressed as simple fractions, you can do that instead: “one and a half” for 1.5.

If you absolutely want to write it out, then you'll have to do it the way they are spelt: “one point five”, “five point zero”. You can use oh instead of zero (“five point oh”, “oh point five”) to reflect an informal spoken style.

@Alenanno: It's hard to imagine a context where you want the precision of a noninteger number that can't be approximated well enough using half/third/quarter/etc., and where writing the actual digits/decimal point would be out of place. Style, readability, brevity - lots of reasons to favour the latter.
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FumbleFingersJun 4 '11 at 15:59

3

@FumbleFingers: One example of context could be the dialogue of an android character [such as Commander Data from the popular television series "Star Trek: The Next Generation"] who regularly says things like "...three point fourty-two light years..." as part of casual conversation.
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Randolf RichardsonJun 4 '11 at 16:12

2

I think (?) in England you wouldn't say "oh" but you might say "naught".
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GEdgarJun 4 '11 at 16:19

2

@F'x and GEdgar: What would the English know about English??! :-)
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Peter K.Jun 4 '11 at 17:06

2

@GEdgar: They do. Naught would be spelled "nought".
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James PoulsonJun 4 '11 at 17:40

Reading the decimal. In this method you read the part of the number left of the decimal point as a normal integer, then the word “point”, followed by the numbers to the right of the decimal point. They are read either as individual digits, or sometimes in pairs as two-digit numbers. Examples:

1.5 = one point five

5.0 = five point zero or five point oh

1.05 = one point zero five or one point oh five

1.21 = one point two one or one point twenty-one

17.365 = seventeen point three six five or seventeen point three sixty-five

3.9715 = three point nine seven one five or three point ninety-seven fifteen

This method of reading out decimal numbers is the most common these days, and would be how how almost anyone would read out a decimal number when speaking out loud.

Converting to a decimal fraction. In this method, the part to the right of the decimal point is treated as a fraction whose numerator is an integer represented by the digits to the right of the decimal point and whose denominator is a power of ten, depending on the number of digits:

This method is a pretty unusual and old-fashioned way of reading out decimal numbers, although it is unimpeachably correct for the standpoint of historical tradition. You would generally only find this method of reading or spelling out in very formal situations or where a very strict and old-fashioned style guide is in place.

The latter method is the way we use to read decimals, Haven't noticed how old-fashioned we're acting!
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GigiliJun 4 '11 at 17:42

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I have to say -- I disagree. At least where I'm from, we rarely if ever use the first form, and use the second form almost exclusively. I think this depends on where you're from.
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Billy ONealJun 4 '11 at 18:04

@Billy I could see how you might get away with the second method most of the time but it seems incredibly unwieldy if you have more than 2 digits after the decimal point. Do people really speak of thousands of millionths where you're from?
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nohat♦Jun 4 '11 at 20:23

@nohat: Ah.. I see. Thousandths, yes, I see that regularly. Millionths I don't though. Anything more than three decimals usually gets read out as "[long series of digits] EEEE [some number designating a power of ten]". (Here saying eeee comes from the "ee" button on calculators)
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Billy ONealJun 4 '11 at 20:29

I'd like to add a third option to @nohat's answer. In scientific settings you can often eliminate the decimal altogether by changing the units. For example, if I was reading this:

We found that there was a .25 meter separation between component abc and component xyz.

I would probably read this as "We found that there was a twenty five centimeter separation between ..."

Putting everything into the base SI units is good for writing academic papers because it prevents someone from misleading with different units. (E.g. 700 Billion bailout, 180 Million bonuses) However, when speaking the less awkward pronunciation usually makes removing the decimal helpful for people to understand. People don't try to reconstruct long strings of digits in their head, their eyes simply glaze over and stop paying attention.

All the answers have been quite good. I suspect that this is determined locally, to some degree, a lot by context (are you rattling lots of figures off, or is this a formal presentation, etc.) and by the required formality / style guide.

I'll throw a few more into the mix (keep in mind my midwestern American English sense):

1.5 becomes one and one half or one and a half. More common: one point five. (.25 becomes quarter, .75_ becomes three quarters.)

5.0 becomes five, five-all, five-even, or five point oh and occasionally five point zero.

2.79 becomes two point seventy-nine or two point seven nine. Rarely any other way is used, but if I had to go "old fashioned", it'd be two and seventy-nine hundredths (as in writing checks). If your context has established an expectation of a decimal point, then you might hear two seventy-nine.

$6.34 becomes six dollars and thirty-four cents in formal use, but often said six thirty-four unless the meaning would be overly ambiguous (e.g.: 100.14 would be said one hundred dollars and 14 cents, not one hundred and fourteen.)

11:46 becomes eleven forty-six almost all the time. Only when there are no minutes to report does "o'clock" occasionally get added. Occasionally half-past (:30), five till (:55).

Side note: Occasionally nought and ought are heard for zero. It raises more eyebrows and question-marks when it is used, though, so best to avoid it.

Occasionally nought and ought are heard for zero, I think that depends on where you're coming from; nought is quite common around here in Sydney, although I personally don't particularly quite liked it.
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Lie RyanJun 5 '11 at 13:28

"One and five tenth"? What is that "tenth"? I thought they could only be expressed saying the numbers, like F'x said...
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AlenannoJun 4 '11 at 14:51

3

No, tenth is the denominator of a fraction, so you can say "one and five tenthS". But better to say "one and a half"
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TimLymingtonJun 4 '11 at 15:20

2

@Gigili, @TimLymington is correct. Your answer is right except for the fact that the first place after the decimal is the TENTHS place not the TENTH place. TENTHS, HUNDREDTHS, THOUSANDTHS, TEN-THOUSANDTHS, etc.
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nohat♦Jun 4 '11 at 16:32

2

@TimLymington - "One and a half" and "one and five tenths" mean different things, even though they specify the same number. The latter suggests that your measurement is accurate to a tenth; with the former, who knows.
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Rex KerrJun 4 '11 at 16:34

3

4.1 is "four and one tenth", not "tenths", just as elsewhere in English.
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GEdgarJun 4 '11 at 16:40